That looks really interesting. Nice work. I've not heard of this before, what are the benefits and drawbacks of a design like this?
I like the Ripole's compact size, the seamless blending with my ESL mains, and especially the clean non-boomy sound.
Before I built the Ripoles I was running a sealed box sub with a Dayton RS 12, which sounded like it only played a single note and boomed like crazy. Of course; there was actually nothing wrong with the sealed box sub-- just that its monopole radiation pattern excited my room's resonance; giving the dreaded 'one-note-boom'. In contrast; the Ripole's cardroid radiation pattern nulls the off axis output, and I believe for that reason, it doesn't excite my room's resonance.
The downside is that, while they have good efficiency at mid bass frequencies; the dipole roll off limits how loud they can play on the bottom end, as it doesn't take a lot of amp power to bottom out the woofers at X-max. That's why I built a pair of them, not just one. Another downside is the chamber resonance, which generates sharp, loud peak in the output centered somewhere between 200-300 Hz.
For DIY'ers, it would take some smarts and a bit of trial and error to design a notch filter to tame the Ripoles characteristic chamber resonance. My website lists components and values for a generic passive notch filter which would get you in the ballpark, but it would still take measurement capability and trial and error to fine tune a notch filter. Passive filters aren't my forte so I took the easy way out and just set the crossover below the chamber resonance using a DSP crossover and steep filter slope, and then tuned it by ear with parametric EQ's until is sounded right.
I've grown to love the sound of my Ripoles but I make it a point never to recommend them to others because not everyone likes dipole bass-- which is clean and musical but doesn't pump out chest thumping bass like monopole subs do, and they aren't very efficient either. There was one guy on the DIY Forum who took every opportunity to bad mouth Ripoles, even though many others there liked them. There doesn't seem to be any middle ground opinions.
I have very clean transmission line bass in my hybrid ESL mains and I didn't want the Ripoles stepping on them, so I chose to cross in the Ripoles at a very low 60Hz using a 48db/octave digital filter and apply a 6db/octave shelving EQ to offset the dipole roll off. Most music doesn't have a lot of content below 60Hz anyway so I seldom actually hear the Ripoles playing. But if I switch off their amplifier; the foundation of the music disappears and the Ripoles' absence is then quite apparent.
And if I play a tune with profoundly low bass content, like Bela Fleck's "Flight of the Cosmic Hippo"-- then I go WOW! --that's LOW and CLEAN.